Position all selected vertices to the same place
Just found an answer from: http://blenderartists.org/forum/showthread.php?167457-Transform-multiple-vertices-flatten-snap-to


  There are two methods that I usually use
  
  
  select the vertices and press S Z 0 to put all the vertices in the same z-position, then move then set the 'Median Z'
  value in the transform properties.
  put the cursor in the wanted z-position, select the wanted vertices and set pivot to 3d cursor, then press  S Z 0